This invention relates generally to implantable stimulators and, more specifically, to implantable pacemakers, cardioverters and defibrillators.
Over the years, numerous methods have been proposed for pacing the heart in an attempt to interrupt tachycardias. These include such pacing modalities as overdrive pacing, burst pacing, autodecremental overdrive pacing, and others. These pacing modalities have been formulated to interrupt aberrant reentrant conduction which may lead to sustained tachycardias in one or more chambers of the heart.
It has been proposed that tachycardias could be prevented or interrupted by the use of multi-site cardiac pacing. One early example of multi-site cardiac pacing to terminate or prevent tachyarrhythmia is disclosed in U.S. Pat. No. 3,937,226 issued to Funke. In this device, a number of small surface area pacing electrodes are provided, each coupled to a separate output circuit and amplifier. The disclosed device is equivalent to five or more separate cardiac pacemaker output circuits of conventional design, all adapted to be triggered to pace simultaneously at various locations around the heart. It is hypothesized that by stimulating simultaneously at locations spread around the heart, synchronous with a sensed QRS complex, arrhythmias could be prevented by producing a more nearly simultaneous depolarization of cardiac tissues.
In contrast, fibrillation has generally been treated by means of high energy shocks, which, in the context of implantable anti-arrhythmia devices, are applied by means or large surface area electrodes, including an electrode on or in the chamber to be defibrillated. The high energy level is employed in order to simultaneously depolarize the bulk of the heart chamber to be defibrillated, which will include tissues in all stages of the depolarization-repolarization cycle at the time the pulse is delivered.
In the context of atrial fibrillation, a proposed pacemaker/defibrillator is disclosed in PCT Application No. US 92/02829, Publication No. WO 92/18198 by Adams et al, incorporated herein by reference in its entirety. In this reference careful synchronization of the high voltage atrial defibrillation pulse to the ventricles to avoid induction of ventricular tachycardia or fibrillation is discussed. Delivery of an atrial defibrillation pulse at an inappropriate time may induce ventricular arrhythmias, including ventricular fibrillation.
Use of pacing pulses delivered at multiple sites within the atria to prevent the occurrence of atrial tachyarrhythmias including atrial flutter, which may in some cases progress to atrial fibrillation, has been investigated. For example, the article "Prevention of Atrial Tachyarrhythmias Related to Advanced Interatrial Block by Permanent Atrial Resynchronization, by Daubert et al, Pace, Vol. 14, P. 648, 1991, discloses the use of synchronized pacing pulses delivered to the right and left atria to prevent onset of atrial tachyarrhythmias.
Recently, the theoretical possibility of employing low energy pacing level pulses (i.e. less than 0.05 joules) to terminate fibrillation has been explored. For example, in the recent article "Regional Control of Atrial Fibrillation by Rapid Pacing in Conscious Dogs", by Allessie et al, published in Circulation, Volume 84, No. 4, October 1991, pages 1689-1697, the ability of pacing pulses to capture a small area of fibrillating atrial tissue, if applied during a specified time interval synchronized to the sensed depolarization waveform at the pacing electrode site has been demonstrated. However, the depolarization wavefront created by such pulses does not propagate through the entire chamber, due to the varying polarization states of the tissue surrounding the stimulation site.
Delivery of pulse bursts to the atrium in the presence of atrial fibrillation is disclosed in U.S. patent application Ser. No. 08/082,327, now U.S. Pat. No. 5,356,425, for a "Method and Apparatus for Treatment of Atrial Fibrillation and Flutter, filed Jun. 24, 1993 by Bardy et al. In this device, pulse bursts are delivered in response to detected high ventricular rate, in patients having persistent or frequent atrial fibrillation. The pulse bursts are synchronized to individual depolarizations to stimulate the nerves within the AV nodal fat pad, to produce partial heart block and thus reduce ventricular rate, if required.
Delivery of high frequency pulse bursts to the atrium is also known to induce atrial fibrillation, unless synchronized to atrial depolarizations to assure that the pulse bursts occur within the refractory period of the atrium. This effect is discussed in U.S. patent application Ser. No. 08/086,278, now U.S. Pat. No. 5,334,221, for a Method and Apparatus for Treatment of Angina, filed Jun. 30, 1993 by Bardy, which discloses a device which provides pulse bursts to the atrium, synchronized to detected atrial depolarizations to stimulate the SA nodal fat pad and reduce the sinus rate of patients who suffer from angina.